Lithium ion secondary batteries are broadly used as energy devices with a high energy density as power sources for portable information terminals such as notebook computers, cell phones, and PDAs (Personal Digital Assistant). For typical lithium ion secondary batteries (hereinafter also referred to simply as “lithium battery”), a group of wound electrodes obtained by layering a cathode, an insulation layer, an anode, and an insulation layer in this order and coiling the same, or a group of laminated electrodes obtained by layering a cathode, an insulation layer, an anode, and an insulation layer, has been used. As an active material for an anode, a carbon material having a multilayer structure capable of intercalating (formation of a lithium intercalation compound) a lithium ion between layers and releasing the same is mainly used. As an active material for a cathode, a lithium-containing metal complex oxide, and as an insulation layer, a polyolefin porous film is mainly used. Such lithium ion secondary batteries have high battery capacities and output powers as well as superior charge and discharge cycle performances.
Although lithium ion secondary batteries are at a high level in terms of safety, further improvement of safety has been demanded in view of their high capacities and high power. For example, when lithium ion secondary batteries are overcharged, there is possibility of heat generation. Further, heat may also be generated by occurrence of an internal short-circuit. Moreover, since lithium ion secondary batteries contain a nonaqueous electrolyte containing an organic solvent, there is the possibility that the generated heat will cause chemical degradation of the organic solvent to generate a gas, leading to a trouble such as an increase in an internal pressure of a battery or the like.
Currently, further improvement of the safety of lithium ion secondary batteries is intended by cutting off a current inside the batteries to suppress heat generation, when the lithium ion secondary batteries are overcharged or the like. Examples of means for improving safety include: (1) a method of utilizing a mechanism, such as a safety valve provided in a sealing plate, which detects an internal pressure of a battery and cuts off the current; (2) a method in which a sealing plate is provided with a component composed of a PTC (Positive temperature coefficient) element of which electrical resistance increases according to the heat generation in the battery, and the current is cut off when the PTC element becomes a nonconductor; and (3) a method in which an insulation layer meltable corresponding to heat generation in the battery is used, and when the insulation layer melts down, movement of a lithium ion between a cathode and an anode is inhibited so that the current is cut off.
Meanwhile, as another method for cutting off a current to suppress heat generation, an electrode provided with a PTC layer has been proposed by Japanese National-Phase Publication (JP-A) No. 2002-526897, Japanese Patent Application Laid-Open (JP-A) No. H10-50294, and JP-A No. 2009-176599. A PTC layer indicates, similar to a PTC element, a layer having a function to increase the electrical resistance (direct current resistance) corresponding to heat generation of a battery. The electrode (at least one of a cathode and an anode) according to JP-A No. 2002-526897, JP-A No. H10-50294, and JP-A No. 2009-176599 is a layered body in which a cathode active material layer or an anode active material layer, a PTC layer, and a current collector are layered in this order. The PTC layer according to JP-A No. 2002-526897 is formed by mixing and heating an electrically conductive particle (carbon particle) and a binder resin (a polymer mixture of a low-density polyethylene and ethylene-vinyl acetate), forming the obtained mixture into a sheet, and then heat-treating the obtained sheet and layering the same on a current collector.
According to JP-A No. H10-50294, a PTC layer is formed by mixing electrically conductive microbeads and a matrix resin (a poly(vinylidene fluoride) powder) in an organic solvent to prepare a paste, coating the paste on a surface of a cathode current collector and a surface of an anode current collector, and drying the same. In such a PTC layer, an electrically conductive network is formed by mutual contact among the electrically conductive particles. With respect to the electrically conductive network, when a matrix resin expands due to overheating, the contact among electrically conductive particles is broken so as to cut off the current.
A PTC layer according to JP-A No. 2009-176599 is formed by preparing a paste by dispersing an electrically conductive particle (carbon particle) and a polymer particle (polyethylene particle) in water, coating the paste on a surface of a cathode current collector, and then drying the same.